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Article

Characteristics of Lodging Resistance of Wheat Cultivars from Different Breeding Decades as Affected by the Application of Paclobutrazol Under Shading Stress

by
Dianliang Peng
1,
Haicheng Xu
1,*,
Zhen Guo
1,
Wenchao Cao
1,
Jingmin Zhang
1,
Mei Liu
1,
Xingcui Wang
1,
Yuhai Tang
1 and
Tie Cai
2,3,*
1
Shandong Provincial University Laboratory for Protected Horticulture, College of Agronomy, Weifang University of Science and Technology, Weifang 262700, China
2
College of Agronomy, Northwest A&F University, Yangling 712100, China
3
Key Laboratory of Crop Physi-ecology and Tillage Science in Northwestern Loess Plateau, Ministry of Agriculture and Rural Affairs, Northwest A&F University, Yangling 712100, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(8), 1848; https://doi.org/10.3390/agronomy15081848
Submission received: 8 July 2025 / Revised: 22 July 2025 / Accepted: 30 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Mechanisms and Pathways for Enhancing Crop Stress Resistance, Yield, and Quality)
Browse Figures
Versions Notes

Abstract

Low solar radiation, caused by climate change or dense planting patterns, now limits wheat production. Although wheat breeding has increased lodging resistance and yield potential through the introduction of dwarfing genes, it still reduces wheat yields. Few studies have been conducted to clarify the lodging sensitivity to shading of different-era wheat cultivars in China’s Huang-Huai-Hai region, as well as the characteristics of lodging resistance as affected by paclobutrazol under shading stress. To address this gap, the experiment included two wheat cultivars released in different decades, grown under shade and treated with or without paclobutrazol. The results showed that reductions in filling degree and lignin content, together with increases in length of the basal internode and gravity center height, markedly reduced the section modulus and breaking strength of shaded wheat culms. These changes impaired lodging resistance and raised lodging risk. However, paclobutrazol application effectively reduced lodging incidence and increased wheat yield under shading stress. Furthermore, these responses were more pronounced in the old cultivar (YZM) than in the modern cultivar (S28). This indicates that the culm mechanical parameters of the old cultivar were more shade-sensitive than those of the modern cultivar. Moreover, shading downregulated the relative expression levels of key genes associated with lignin biosynthesis to decrease the activities of key enzymes, thereby inhibiting the biosynthesis and deposition of lignin in culms to increase the risk of wheat lodging. Paclobutrazol application alleviated the inhibitory effects of shading on lignin biosynthesis, thereby strengthening culms and enhancing lodging resistance. These findings may provide a basis for exploring cultivation regulation methods to enhance wheat lodging resistance under overcast and low-sunshine conditions, and to offer guidance for the breeding of wheat cultivars with lodging resistance and shade tolerance.

1. Introduction

Lodging, which refers to the permanent tilting or bending of wheat culms from their vertical position due to various endogenous and exogenous elements [1], is a significant factor that restricts wheat yields and quality on a global scale [2,3]. In the 1960s, breeders introduced the Rht dwarfing gene to shorten plants, enhance lodging resistance and boost yield by raising the harvest index [4]. Nevertheless, multiple studies have demonstrated that overuse of dwarfing genes to reduce plant height can compromise yields. An optimal height range of approximately 70–100 cm is crucial for maximizing productivity [2,4]. Since modern cultivars already fall within this range, further height reductions may offer limited value for future breeding efforts aimed at improving lodging resistance while maintaining yield potential [5,6].
To safeguard China’s food security, it is essential to continuously boost wheat productivity per unit area. Elevating plant density to boost spike numbers per unit area serves as the chief agricultural strategy for further enhancing wheat yield [3,7]. However, dense planting deteriorates the light environment, characterized by reduced light levels and a diminished red-to-far-red light ratio, among other factors [8,9]. Another crucial point is that, in recent years, influenced by global climate change, wheat has often faced prolonged rain and overcast conditions during its growth, signifying reduced solar irradiation [10]. In the Huang-Huai-Hai grain-producing area, which represents 75% of China’s total wheat production, annual sunshine hours have declined by 3.74–9.22 h overall, and by 2.98–3.67 h specifically during the winter wheat growth cycle [11]. Low light has thus become a limiting factor in current wheat cultivation and production [12].
Wheat lodging mainly results from insufficient mechanical resistance and bending strength in the basal internodes [3,8]. Lignin, an important structural substance in the secondary cell walls of higher plants, not only plays a role in plant growth and development but is also closely related to the formation of plant morphogenesis and tissue mechanical strength [3,6,13]. As early as 1928, researchers identified that a lack of lignin in wheat culms contributes to lodging [14]. Since then, numerous follow-up studies have indicated that lignin enhances the load-bearing capacity of plant culms [15,16,17]. The synthesis and accumulation of lignin in wheat culms are influenced by both the genetic traits of cultivars and environmental factors [8,18,19,20]. The enhanced lodging resistance in the rice dwarf mutant pex results from the high accumulation of lignin in the culm, driven by high expression of genes involved in the lignin synthesis pathway [21]. Plant growth regulators have emerged as a crucial technique to ensure or boost crop yield [22], enhance crop quality, and improve stress resistance [23,24]. Triazole compounds, such as paclobutrazol (PP333) or uniconazole, regulate plant growth by restricting gibberellin formation, boosting cytokinin levels, and reducing culm elongation via their effects on the isoprenoid pathway [25,26]. Specifically, large-population cultivation decreases lignin accumulation in cytoderm, which weakens the bending resistance of culm tissue, thereby lowering anti-lodging performance [27]. Meanwhile, higher nitrogen application rates increase lodging risk in wheat due to insufficient lignin buildup [28,29]. Shading boosts GA levels and upregulates gibberellin metabolism enzyme genes in soybean culms, stimulating internode elongation. Conversely, paclobutrazol application suppresses elongation and lowers GA levels [30]. Previous studies have indicated that paclobutrazol can markedly enhance the stress tolerance and adjustment capabilities of crops to adverse environments [31,32]. Paclobutrazol application has been demonstrated to elevate lignin deposition and boost the activities of lignin-associated enzymes in maize and wheat [33,34].
Although previous researchers have conducted numerous studies on the relationship between wheat’s resistance to lodging and internode characteristics [3,8,9,10,11,35], the responses of culm traits and lignin synthesis of wheat cultivars from different breeding decades to low light conditions have rarely been reported. Therefore, the aim of this study was (i) to clarify the lodging sensitivity to shading of wheat cultivars released in different eras in the Huang-Huai-Hai wheat region of China, and (ii) to identify characteristics of lodging resistance in these cultivars as affected by paclobutrazol under shading stress. The results obtained are expected to provide a basis for developing cultivation strategies to enhance wheat lodging resistance under overcast or low-sunshine conditions, and to offer guidance for breeding wheat cultivars with lodging resistance and shade tolerance.

2. Materials and Methods

2.1. Plant Material and Experimental Design

The experiment was conducted from 2020 to 2022 at B.L. Experimental Farm (36°85’ N, 118°79’ E), Weifang U&T University, Shouguang, Shandong, China. The region has a warm temperate, semi-humid climate with continental monsoon characteristics. Figure 1 illustrates the climate data for these growing seasons. The soil at the experimental site was a loam. The pH, available nitrogen, available phosphate, and available potassium in the topsoil (0–20 cm) were detected to be 6.85, 76.9 mg kg−1, 22.9 mg kg−1, and 81.6 mg kg−1, respectively.
Two winter cultivars released in different decades from the Huang-Huai-Hai wheat region of China were used: Youzimai (YZM, an old genotype with a plant height of approximately 109 cm, released in 1950), Shannong28 (S28, a modern genotype with a plant height of approximately 75 cm, released in 2014). The treatments consisted of two shading treatments, natural light and shading, and two PP333 (Sichuan Guoguang Agrochemical Co., Ltd., Sichuan, China) application treatments, 0 and 150 mg L−1 PP333, which were determined based on our previous studies [33]. The treatments were as follows: natural light with 0 mg L−1 PP333 treatment (NL), natural light with 150 mg L−1 PP333 treatment (NP), shading with 0 mg L−1 PP333 treatment (SH), and shading with 150 mg L−1 PP333 treatment (SP). The shading treatment involved using a black shade net to block 26.36% of light. The shading period extended from the jointing stage to maturity (GS30-GS92) [36]. The shade net was set up at 3 m above the ground to ensure good ventilation and light transmission around it. The 0 and 150 mg L−1 PP333 were sprayed onto wheat leaves at the early jointing stage (GS30). Each concentration was applied at 500 mL m−2 over two consecutive days from 17:30 to 18:30 (after sunset), using 0.2% (v/v) Tween-20 as the surfactant. Wheat was sown on 9 October 2020 and 5 October 2021, with a planting density of 210 plants m−2.
The experiment used a split-split-plot design, with cultivars as the main plots, shading as the sub-plots, and PP333 applications as the sub-sub-plots. For each treatment, three replicates of wheat plants were fitted with nylon nets (+net; 25 cm × 25 cm mesh, 0.2 cm rope diameter) near the jointing stage (GS33), without altering the canopy structure. These nets allowed plants to grow through them, preventing natural lodging, to assess lignin accumulation and breaking strength of the basal 2nd internodes. Another three replicates of each treatment were left to grow naturally (−net) to investigate lodging behavior, including the timing, angle, and percentage of lodging. Other planting management referred to the high-yielding and efficient farming technology regulations for wheat proposed by Yu [37].

2.2. Sampling, Lodging, and Yield Investigation

Samples were collected at three growth stages: jointing (GS33), anthesis (GS65), and dough (GS82). Twenty samples from the second internode of wheat culms were rapidly excised, stored at −80 °C for analysis of lignin content, activities, and gene expression of phenylalanine aminotransferase (PAL), cinnamoyl-CoA reductase (CCR), 4-coumarate-CoA ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD), as well as endogenous phytohormones. The initial time of lodging for each treatment was documented. The lodging angle was measured as the angle of lodging from vertical position at the time of lodging. The lodging rate was determined according to the area of each plot that was lodged using Equation (1) [3,33]:
Lodging rate (%) = (lodged area of the plot/total plot area) × 100%.
The crop yield was assessed within a 2 m2 area for each plot. The number of spikes within each quadrat was recorded. Twenty wheat spikes were indiscriminately chosen to measure the grains per spike. Additionally, 1000 grains were arbitrarily chosen to determine the 1000-grain weight. Each treatment was replicated three times [3,27,33].

2.3. Measurements of Mechanical Traits, Lignin Synthesis, and Hormones in Culms

2.3.1. Determination of Mechanical Traits

The mechanical parameters of culms were assessed using the methods as follows [8,38,39]. Plant height was the total above-ground length of a single wheat plant. The center of gravity height (CGH) refers to the distance from the culm base to the balance fulcrum. Basal internode length proportion (BIL), filling degree (FD) and section modulus (SM, mm3) were determined using Equations (2)–(4), as follows:
BILP (%) = (the length of basal 1st and 2nd internode/plant height) × 100%.
FD (mg cm−1) = dry weight of 2nd internode/length of the internode.
SM (mm3) = πD3/32 (1 – (d4 – D4))
where D and d denote the outer and inner diameters of culms, respectively. The basal second internode with non-leaf sheath was placed in a support frame (30 cm high, 5 cm spacing). A culm breakage tester (YYD-1A, Hangzhou, China) applied uniform pressure to the midpoint of the internode. The breaking strength (N) was recorded when the culm fractured. The lodging resistance index (LRI) was determined using Equation (5),
LRI = FL/4MgH
where F represents the culm breaking strength, L is the standard moment, M denotes the fresh weight of a single culm, g is the acceleration due to gravity, and H is the CGH.

2.3.2. Determination of Lignin Content

A test kit (Lignin Test Kit, Shanghai, China) was employed to measure the lignin accumulation [8]. For each treatment, 2 mg of dried powdered culm was mixed with 500 μL sulfuric acid and 20 μL perchloric acid in a 2.5 mL tube, sealed, and heated at 80 °C for 40 min with periodic shaking. After cooling, 995 μL NaOH was mixed with 0.5 mL of the sample. Lignin values were determined by incubating 200 μL of the sample solution in a 96-well plate, with absorbance at 280 nm measured using ELISA (SpectraMaxi3x, Molecular Devices, San Jose, CA, USA). The calculation Formula (6) for lignin was as follows:
Lignin content (mg g−1) = 0.075 × (ΔA − 0.0068) − W × T
where ΔA represents the absorbance, W denotes the dry sample weight, and T stands for the dilution ratio.

2.3.3. Determination of Activities of Enzymes Involved in Lignin Biosynthesis

The activities of enzymes associated with lignin biosynthesis were assessed by pulverizing 200 mg of fresh 2nd internode culm. The activities of PAL, 4CL, and CAD were evaluated according to the experimental methods of predecessors with minor modifications [40,41,42]. The CCR activity was measured by an ELISA kit (CCR Test Kit, Shanghai, China).

2.3.4. Determination of Expression of Genes Encoding Key Enzymes

Total RNA was extracted using a test kit (Tiangen, Beijing, China). The cDNA was synthesized by reverse transcription Kit Magic1st cDNA Synthesis Kit. Primers were designed using Primer 5.0, with TaActin serving as the reference gene. Real-time fluorescence quantitative PCR was carried out with the QuantFast SYBR Green qPCR Super-Mix kit (Magic-Bio Co., Ltd., Hangzhou, China) and a QuantStudio 3 PCR system (Thermo Fisher Scientific, Waltham, MA, USA). The relative expression levels of the genes were assessed using the 2−ΔΔCt method to analyze the test results [43].

2.3.5. Determination of Endogenous Hormones of Culms

Approximately 0.5 g of stem tissue was sampled for extraction and purification of endogenous hormones. Hormone contents were measured by high-performance liquid chromatography according to the methods reported by Yang et al. and Cai et al. [44,45].

2.4. Statistical Analyses

Mean and standard errors were assessed for individual measurements. Variance analysis was conducted using the DPSv7.05 Data Processing System (Hangzhou RuiFeng Information Technology Co., Ltd., Hangzhou, China). The least significant differences (LSD) test was employed to assess differences between treatments at the 0.05 probability level. Pearson’s correlation coefficients were calculated to assess the relationships among lignin accumulation, mechanical parameters of stems, and yield. Microsoft Excel 2010 (Microsoft Corp., Redmond, WA, USA) and SigmaPlot 12.5 (Systat Software Inc., Richmond, CA, USA) software were used for data analyses and post-processing.

3. Results

3.1. Grain Yield and Yield Components

Cultivar, shading, and PP333 significantly affected spike number per m2, grain number per spike, 1000-grain weight, and grain yield (Table 1), with the interaction between cultivar and shading also significantly influencing grain number per spike and grain yield. Over two years, the modern cultivar S28 consistently outperformed the old cultivar YZM in these yield components, except for grain number per spike (Figure 2A–H). Compared to natural light, shading starting at the jointing stage reduced spike number per m2 (Figure 2A,B), grain number per spike (Figure 2C,D), and 1000-grain weight (Figure 2E,F) by 10.49%, 14.65%, and 20.19%, respectively (mean of both cultivars). Under shading, the modern cultivar S28 exhibited significantly lower reductions in these yield components than the old cultivar YZM. Under both natural light and shading, PP333 significantly increased spike number per m2 and grain number per spike but decreased 1000-grain weight for both cultivars.
Flat cultivation with support nets (+net) completely prevented lodging and led to a substantial increase in grain yield (Figure 2G,H). In contrast, lodging occurred in flat cultivation without support nets (−net). Compared to natural light, shading significantly reduced grain yield (+net) by 34.59% in the modern genotype and by 48.43% in the old genotype (mean of both years). Under normal light, PP333 application had no significant effect on grain yield with support nets (+net). However, PP333 significantly increased grain yield in both cultivars under normal light and shading stress without support nets (−net), with the modern cultivar increasing by 6.81% (mean of both years) and the old cultivar by 19.49% (mean of both years).

3.2. Incidence of Lodging in the Field

Cultivar, shading, and PP333 significantly influenced lodging angle and lodging rate (Table 2). Under natural light, the modern cultivar S28 remained lodging-free across both years, whereas the old cultivar YZM exhibited severe lodging. Under shading, YZM displayed both a higher lodging rate and greater lodging angle than S28, with lodging onset occurring earlier in YZM. Notably, the lodging angle and lodging rate of YZM were 3.67 times and 2.79 times higher than those of S28, respectively. Shading significantly increased the lodging angle and rate and caused lodging to occur earlier in both cultivars. Compared with natural light, shading increased the lodging angle by 114.83% and the lodging rate by 134.72% (mean of both cultivars). Regardless of whether under natural light or shading, PP333 application significantly reduced the lodging angle and rate and delayed lodging occurrence. Specifically, compared with NL treatment, the NP treatment reduced the lodging angle by 35.69% and the lodging rate by 47.26%. Similarly, compared with SH, SP reduced the lodging angle and the lodging rate by 27.99% and 21.96%, respectively (mean of both cultivars).

3.3. Culm Morphological Characteristics

Plant height, gravity center height (GCH), and basal internode length proportion (BILP) were significantly affected by cultivar, shading, and PP333 application (Table 3), and the interaction among these factors significantly influenced plant height. Across the two growing seasons, the old cultivar YZM exhibited higher values for these parameters compared to the modern cultivar S28 under all treatments. Shading significantly increased plant height (Figure 3A,B, the upper bar chart), GCH (Figure 3A,B, the lower bar chart), and BILP (Figure 3C,D) in both wheat cultivars. Specifically, compared with NL treatment, SH treatment increased these parameters by 8.91%, 15.16%, and 16.98%, respectively (mean of both cultivars). Similarly, compared with NP, SP increased these parameters by 11.90%, 19.75%, and 17.07%, respectively. PP333 application generally reduced these parameters, except for basal internode length proportion in YZM. Compared with NL, NP reduced plant height, GCH, and BILP by 6.36%, 9.09%, and 35.40%, respectively (mean of both cultivars). Similarly, compared with SH, SP reduced these parameters by 3.78%, 5.45%, and 9.40%, respectively.

3.4. Filling Degree and Section Modulus

During growth, the filling degree (FD) of wheat basal 2nd internode first increased and then declined (Figure 4A–D), whereas the section modulus (SM) steadily improved over the entire growth period (Figure 4E–H). Significant differences in FD and SM were observed among treatments. Cultivar S28 exhibited higher stem FD and SM than YZM. Shading significantly reduced these parameters in both cultivars. Compared with natural light, shading reduced the FD and SM by 33.10% and 44.51%, respectively, for YZM (mean of both years), and by 30.13% and 36.78%, respectively, for S28. Under normal light, PP333 treatments significantly improved these parameters, except for the anthesis stage of S28 (p > 0.05). Specifically, compared to NL, the FD and SM under NP were substantially elevated by 18.89% and 27.54%, respectively (mean of both cultivars). Under shading, PP333 application did not significantly affect these parameters, except for increasing the filling degree at the anthesis stage for both cultivars.

3.5. Breaking Strength and Lodging Resistance Index

During the development of the culm, the breaking strength (BS) of wheat 2nd internode first increased and then declined (Figure 5A–D). Meanwhile, the lodging resistance index (LRI) gradually diminished throughout the development (Figure 5E–H). Across both growing seasons, the old cultivar YZM consistently had lower BS and LRI than the modern cultivar S28 under all treatments. Shading significantly reduced these parameters for both cultivars. Specifically, compared with natural light, shading decreased the BS and LRI by 33.10% and 30.61%, respectively, for S28 (mean of both years), and by 37.98% and 35.69%, respectively, for YZM. Overall, compared to NL, SH reduced these parameters by 34.40% and 34.82%, respectively (mean of both cultivars). Similarly, compared to NP, SP reduced them by 32.29% and 30.42%, respectively. Regardless of whether under natural light or shading, PP333 significantly enhanced the breaking strength and lodging resistance index. Specifically, compared with NL, the BS and LRI under NP increased significantly by 16.80% and 28.18%, respectively (mean of both cultivars). As compared to SH, the BS and LRI under NP increased by 23.06% and 36.83%, respectively. The effect of PP333 on these parameters was stronger under shading than under normal light conditions.

3.6. The Accumulation of Lignin

Cultivar, shading, and PP333 had significant effects on the lignin content (Table 3). The lignin content of basal 2nd internode increased incrementally throughout the developmental stages, with rapid accumulation occurring from jointing to anthesis stage. These trends were consistent for both cultivars across the two growing seasons (Figure 6A–D). Lignin accumulation differed among cultivars and treatments. At the dough stage, the modern cultivar S28 had 28.52% more lignin accumulation than the old cultivar YZM (mean of both years). Shading significantly decreased lignin content in the stems of both cultivars. Compared to NL, SH reduced total lignin accumulation by 28.67% for YZM and 22.59% for S28 (mean of both years). Similarly, compared to NP, SP reduced lignin accumulation by 28.33% for YZM and 23.35% for S28. Conversely, PP333 significantly increased lignin accumulation across both growing seasons. Specifically, NP treatment increased lignin accumulation by 9.34% for YZM and 8.60% for S28 compared to NL (mean of both years). Meanwhile, compared to SH, SP treatment increased lignin accumulation by 9.99% for YZM and 7.28% for S28.

3.7. Activities of Enzymes Involved in Lignin Biosynthesis

During wheat growth, the activity of PAL in wheat 2nd internode dropped progressively (Figure 7A,B). In contrast, the activities of CCR (Figure 7C,D), 4CL (Figure 7E,F), and CAD (Figure 7G,H) first rose and then declined, with peak enzyme activities detected at the anthesis stage (Figure 7C–H). The activities of PAL, CCR, 4CL, and CAD showed significant differences under different treatment conditions at jointing and anthesis stage. In general, shading notably decreased these enzyme activities compared to natural light. Specifically, compared with NL, SH diminished the mean activities of PAL, CCR, 4CL, and CAD by 13.14%, 19.30%, 34.81%, and 30.59%, respectively (averaged across two cultivars). Likewise, compared with NP, SP reduced the mean activities of these enzymes by 16.49%, 26.05%, 30.98%, and 27.56%, respectively. PP333 significantly enhanced the overall activities of PAL, CCR, 4CL, and CAD in both cultivars. Relative to NL, NP elevated the average activities of PAL, CCR, 4CL, and CAD by 12.27%, 18.88%, 16.47%, and 20.65%, respectively (mean of two cultivars). In a similar trend, SP treatment boosted the mean activities of these enzymes by 7.91%, 8.94%, 23.33%, and 25.92%, respectively, when compared to SH.

3.8. Expression of Genes Encoding Key Enzymes in Lignin Biosynthesis

The relative expression level of TaPAL declined progressively from the jointing stage (Figure 8A,B). In contrast, the relative expression levels of TaCCR (Figure 8C,D), Ta4CL (Figure 8E,F), and TaCAD (Figure 8G,H) first rose and then fell, peaking at the anthesis stage. Significant variations in the relative expression levels of TaCCR, Ta4CL, and TaCAD were detected across treatments at the jointing and anthesis stages. The relative expression levels of TaPAL, TaCCR, Ta4CL, and TaCAD reached their peak under NP treatment, with NL, SP, and SH following in that order. Compared to natural light, shading significantly decreased the relative expression levels of TaPAL, TaCCR, Ta4CL, and TaCAD by 23.52%, 32.96%, 41.06%, and 40.44%, respectively (mean of both cultivars). Similarly, compared with the non-PP333 treatment, PP333 application raised the relative expression levels of TaPAL, TaCCR, Ta4CL, and TaCAD by 17.32%, 18.45%, 30.78%, and 29.11%, respectively (averaged across both cultivars). The lignin content and activities of enzymes related to lignin biosynthesis, as well as the relative expression levels of related genes, were significantly affected by light conditions or hormones. Therefore, lignin biosynthesis can be altered by regulating light conditions and applying chemical control measures.

3.9. Endogenous Hormones of Basal Internode

As stem development progressed, the endogenous levels of ZT (Figure 9A,B) and GA7 (Figure 9C,D) in stem tissues generally decreased over time, whereas the levels of IAA (Figure 9E,F) and ABA (Figure 9G,H) initially increased and then decreased. These trends were consistent for both cultivars across the two growing seasons. Moreover, obvious differences in the levels of ZT, GA7, IAA, and ABA were revealed among all treatments at the jointing and anthesis stages. Compared to natural light, shading significantly reduced the levels of ZT, IAA, and ABA by 24.83%, 15.93%, and 32.56%, respectively, while increasing GA7 by 73.59% (mean of two cultivars). The application of PP333 significantly affected hormone levels. Compared with NL, NP increased ZT and ABA levels by 20.51%, and 16.59%, respectively, while reducing GA7 and IAA levels by 26.56% and 14.86%, respectively (mean of two cultivars). Similarly, compared with SH, SP increased ZT and ABA levels by 28.10% and 20.98%, respectively, while decreasing GA7 and IAA levels by 29.03% and 15.85%, respectively (mean of two cultivars).

3.10. Correlation Analysis

The mechanical parameters of wheat culm, including breaking strength and culm lodging resistance index, showed significant negative correlations with basal internode length proportion (p < 0.01; Figure 10A). In contrast, these mechanical properties correlated significantly positively with both the filling degree (FD) and lignin accumulation in the 2nd internode (p < 0.01; Figure 10A). Additionally, the lodging rate exhibited a significant positive correlation with the BILP but significant negative correlations with the FD and lignin accumulation (p < 0.01; Figure 10A). Grain yield correlated significantly positively with stem section modulus (p < 0.01; Figure 10(B1)) and lignin accumulation (p < 0.01; Figure 10(B2)) during the jointing, anthesis, and dough stages.

4. Discussion

In recent decades, the yield of winter wheat in China has increased, mainly due to the rise in grain number per unit area and 1000-grain weight, as documented in previous studies [6,46,47] and corroborated by the findings of this research. Furthermore, it was observed that modern wheat cultivars exhibit a higher spike number per m2 than older cultivars under modern high-yield technologies, which aligns with the results of Zhou et al. and Xiao et al. [48,49] but conflicts with the results of Zhang et al. and Feng et al. [6,47], who reported no noticeable change in spikes per m2. These differences may stem from varying cultivation conditions. Solar radiation is crucial for plant growth and development, and previous studies have shown that shading reduces plant growth and yield in various crops [50,51,52]. In this study, shading decreased spikes per m2, grain number per spike, and 1000-grain weight, thereby reducing the grain yield of both cultivars, which is consistent with previous findings [53]. PP333 application significantly improved spikes per m2 for both cultivars, regardless of shading. However, this contrasts with studies indicating that PP333 has no effect on spike number per m2 [54,55], which may be attributed to differences in application timing and concentration. Additionally, compared to old cultivars, modern cultivars exhibited smaller changes in yield components when subjected to shading or PP333 treatment. To avoid possible lodging, growth retardants are frequently applied to shorten plant height [26,33,55]. In this study, flat planting with support nets (which eliminated lodging) yielded higher grain production than flat planting without support nets (where lodging occurred). However, under normal light, applying PP333 did not significantly affect grain yield. Thus, the effectiveness of growth retardants on grain yield depends on lodging status [33]. For instance, applying retardants in fields with a low risk of lodging may cause yield loss, as shown in studies with dwarfing genes or specific field measures [33,56].
Extreme weather serves as the immediate driver of wheat lodging, with the most severe damage occurring when strong winds and heavy rain coincide [2,27,33,56]. Consistent with this, our trial recorded a higher lodging rate in both wheat cultivars during the 2021–2022 season than during 2020–2021, an increase attributable to higher rainfall frequency and greater cumulative precipitation during the late reproductive phase of the 2021–2022 growing season (Figure 1). Shading significantly affects the morphological characteristics of wheat cultivars. Previous studies have shown that in rice, shading stress increases the lengths of basal internodes and plant height while reducing stem diameter and wall thickness [35,53,57]. In the current study, shading notably elevated plant height, gravity center height, and basal internode length proportion. These findings are consistent with the results from other studies on wheat [3,58]. However, we observed that shading stress reduced the filling degree and section modulus, as well as the breaking strength of stems in both genotypes. Consequently, the lodging angle and lodging rate increased significantly, causing lodging to occur earlier. Correlation analysis revealed that culm filling degree was positively correlated with breaking strength and the lodging resistance index but negatively correlated with the lodging rate. Liu et al. discovered that shading prominently boosted plant height but reduced culm diameter in soybean, thereby increasing lodging risk [59]. Similarly, shading stress can elongate certain internodes while reducing stem diameter and wall thickness, which also heightens lodging risk in some other crops [53,59,60]. From the perspective of material mechanics, the wheat stem can be modelled as an approximately hollow cylinder, and its lodging resistance is governed by stem section modulus and filling degree [3,61,62]. The filling degree of the basal internode first increases due to the accumulation of carbohydrates before anthesis, then declines due to remobilization to developing grains and tissue senescence during grain filling [63,64]. Although the section modulus continues to increase throughout the growth stages, its post-anthesis increment slows, and the gain can no longer compensate for the decline in bending resistance caused by the loss of filling degree, ultimately elevating the risk of lodging [3,61]. Comparable findings were obtained in the present study. PP333 application significantly reduced plant height, center of gravity height, and basal internode length proportion. These reductions were less pronounced under shaded conditions than under normal light, likely due to the shade-avoidance response in wheat, which diminishes the dwarfing effect of PP333 under shading [12,65,66]. The current findings align with prior research indicating that PP333 significantly increases breaking strength and lodging resistance index in various crops [33,35,67]. In the present study, the old cultivar (YZM) exhibited a higher filling degree, section modulus, breaking strength, and lodging resistance index than the modern cultivar (S28). Moreover, the changes in these parameters in response to shading or PP333 were more pronounced in YZM than in S28. These results indicate that the stem mechanical parameters of the old cultivar are more sensitive to shading stress than those of the modern cultivar.
Plant endogenous hormones play a vital role in regulating crop growth and development, as well as influencing plant architecture [45]. In this study, shading significantly reduced the levels of ZT, IAA, and ABA while increasing GA7 levels. Compared to the non-PP333 treatment, PP333 application increased ZT and ABA levels but significantly decreased GA7 and IAA levels. Previous research indicates that higher endogenous gibberellin levels promote internode elongation, thereby increasing plant height and lodging risk [33,57]. However, PP333 inhibits cytochrome P450 monooxygenase activity, reducing GA biosynthesis and suppressing internode elongation [25,68,69]. Compared to normal light, shading significantly reduces the levels of endogenous IAA, Z+ZR, and IP+IPA in rice, which shortens the length of the upper internodes [57]. In soybeans, shading reduces cytokinins and ABA in the stem tissue [70]. ZT can increase cellulose and lignin content in stems, enhancing their mechanical strength. These findings suggest that weak light and exogenous paclobutrazol may regulate internode length and substance accumulation in wheat by altering endogenous ZT, IAA, ABA, and GA levels in basal internodes, affecting cell division and elongation, and ultimately influencing stem filling and lodging resistance [71].
The load-bearing capacity of crop culms is associated with supportive tissues, which are chiefly composed of mechanical cells. The cytoderm, primarily composed of structural materials such as lignin and cellulose, gives the culm bending strength [13,72,73,74]. Studies on lodging resistance in crops like wheat [3,9], barley [75], soybean [16,59], rice [53,76], and buckwheat [24] have showed that lignin, a key cell wall component, is highly correlated with culm sturdiness. Our initial studies also revealed that the formation of lignin deposits in the cell walls of wheat basal internode enhances lodging resistance [33,77]. In the present study, the modern cultivar accumulated more lignin than the old cultivar. Additionally, lignin accumulation was reduced under shading compared to natural light, with the reduction being more pronounced in the old cultivar. This indicates that lignin accumulation in the old cultivar is more sensitive to shading stress. Correlation analysis further suggested that lignin accumulation was significantly positively correlated with both stem filling degree and the lodging resistance index. The synthesis and deposition of lignin in wheat culms are influenced by cultivar differences and external factors [18,19,78]. Shading considerably reduces lignin deposition, with longer shading times causing a greater decrease in lignin and weakening stem mechanics [56,58]. Dense planting reduces lignin deposition in cell walls and weakens the strength of stem mechanical tissues, thereby increasing the risk of wheat lodging [27,79,80]. However, regulating population distribution to optimize the canopy light microclimate can promote the formation and aggregation of lignin in culms, enhancing their resistance to breakage and reducing the incidence of wheat lodging [3]. Previous investigations have demonstrated that paclobutrazol increases stress tolerance and resistance in crops [32,34,81]. In this study, lignin accumulation was significantly higher in the PP333-treated wheat plants than in non-PP333-treated plants, consistent with previous studies [35,67]. Additionally, we found that the effect of PP333 on lignin accumulation was more pronounced under low-light conditions than under normal light conditions.
The lignin biosynthetic metabolism is a complex process, involving many enzymes, among which PAL, CCR, 4CL, and CAD are key enzymes in the lignin synthesis of gramineous crops [27,82]. Previous researchers have carried out extensive studies on the metabolism of lignin and its regulatory mechanisms [59,80,83]. Cheng et al. revealed that in a corn–soybean intercropping system, reducing planting density improves canopy light transmittance and boosts the activities of enzymes associated with lignin biosynthesis in soybean, thereby enhancing lignin accumulation in the stems [84]. Certain research on lodging resistance has indicated that high-density planting decreased the expression of PAL, COMT, CAD, CCR, 4CL, and other genes, which are vital enzymes in lignin biosynthesis, leading to reduced lignin deposition in crop stems [8,85,86]. Light is a major factor regulating plant growth and development, and photosynthesis, which is closely related to light intensity inside the canopy, serves as the material source for plant morphogenesis [87]. Wu et al. discovered that during the jointing stage of rice, shading significantly decreased the expression of genes (PAL, COMT, CAD) involved in secondary cell wall synthesis, leading to a substantial reduction in stem lignin content [3,53]. Similarly, studies on lignin metabolism in soybean stems [59,88] have found that intense shading significantly downregulates the activities of key enzymes in lignin metabolism and the expression of their genes, leading to a decrease in lignin accumulation in the stems, and this effect is independent of the soybean cultivar’s lodging resistance. In the present study, the relative expression levels of TaPAL, TaCCR, Ta4CL, and TaCAD for lignin synthesis were downregulated, while the activities of PAL, CCR, 4CL, and CAD involved in lignin synthesis of wheat culm were significantly inhibited under shading stress compared with natural light, which is aligned with prior research. Additionally, the study found that PP333 application can raise the relative expression levels of genes encoding critical enzymes associated with lignin biosynthesis, thereby enhancing their activities and increasing the deposition of lignin in culms of wheat, consistent with previous studies in some other crops [34,41,67]. In summary, shading stress weakens the expression of genes encoding critical lignin synthesis enzymes at the transcription level, consequently reducing lignin biosynthesis and deposition in wheat culms. In contrast, paclobutrazol application can mitigate the inhibitory effects of shading on lignin biosynthesis metabolism, thereby strengthening wheat culms and enhancing their resistance to lodging.

5. Conclusions

The increased length of the basal internode and gravity center height, along with the decreased filling degree and lignin content in wheat culms, play pivotal roles in reducing the section modulus and breaking strength of wheat stems under shading stress. These changes lead to a reduction in lodging resistance, thereby increasing the risk of lodging. However, the application of paclobutrazol can effectively reduce lodging occurrence and increase wheat yield under shading stress.
Furthermore, the modern cultivar (S28) exhibited higher filling degree, section modulus, breaking strength, and lodging resistance index compared to the old cultivar (YZM). The changes in these parameters in response to shading or paclobutrazol treatment were more pronounced in the old cultivar than in the modern cultivar. This indicates that the culm mechanical parameters of the old cultivar are more shade-sensitive than those of the modern cultivar.
Moreover, shading stress downregulated the relative expression levels of key genes associated with lignin biosynthesis in culms to decrease the activities of key enzymes, thereby inhibiting the biosynthesis and deposition of lignin in culms to increase the risk of wheat lodging. However, paclobutrazol application alleviated the inhibitory effects of shading on lignin biosynthesis, thereby strengthening the wheat culms and enhancing lodging resistance.

Author Contributions

The research presented here was carried out in collaboration between all authors. Conceptualization, D.P., H.X. and T.C.; investigation, Z.G., W.C., J.Z., M.L., X.W. and Y.T.; data curation, D.P., Z.G., W.C. and X.W.; writing—original draft, D.P., H.X. and T.C.; project administration, D.P. and T.C.; writing—review and editing, all authors. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Key R&D Plan of Shaanxi Province (2024NC-YBXM-041), the National Natural Science Foundation of China (No. 32071955), and the Initial Scientific Research Fund for High Level Talent of Weifang University of Science and Technology (KJRC2021001; KJRC2022004).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Accumulated rainfall and mean temperature during the 2020–2021 (A) and 2021–2022 (B) growing seasons.
Figure 1. Accumulated rainfall and mean temperature during the 2020–2021 (A) and 2021–2022 (B) growing seasons.
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Figure 2. Effect of paclobutrazol on spikes number per m2 (A,B), grain number per spike (C,D), 1000-grain weight (E,F), and grain yield (G,H) of wheat under shading condition. The upper bar chart in figure (G,H) represents the grain yield with support nets (+net), and the lower bar chart indicates the grain yield without support nets (−net). NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters (yield with nets marked in black letters and yield without nets marked in blue letters) represent significance at 0.05.
Figure 2. Effect of paclobutrazol on spikes number per m2 (A,B), grain number per spike (C,D), 1000-grain weight (E,F), and grain yield (G,H) of wheat under shading condition. The upper bar chart in figure (G,H) represents the grain yield with support nets (+net), and the lower bar chart indicates the grain yield without support nets (−net). NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters (yield with nets marked in black letters and yield without nets marked in blue letters) represent significance at 0.05.
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Figure 3. Effect of paclobutrazol on plant height (A,B), gravity center height (A,B), and basal internode length proportion (C,D) of wheat under shading condition. The upper bar chart in figure (A,B) represents the plant height, and the lower bar chart indicates the gravity center height. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters (plant height marked in black letters and gravity center height marked in blue letters) represent significance at 0.05.
Figure 3. Effect of paclobutrazol on plant height (A,B), gravity center height (A,B), and basal internode length proportion (C,D) of wheat under shading condition. The upper bar chart in figure (A,B) represents the plant height, and the lower bar chart indicates the gravity center height. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters (plant height marked in black letters and gravity center height marked in blue letters) represent significance at 0.05.
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Figure 4. Effect of paclobutrazol on the filling degree (A–D) and section modulus (E–H) of the 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Different letters in the box on the right side of the scatter plot indicate significance at 0.05.
Figure 4. Effect of paclobutrazol on the filling degree (A–D) and section modulus (E–H) of the 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Different letters in the box on the right side of the scatter plot indicate significance at 0.05.
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Figure 5. Effect of paclobutrazol on the breaking strength (A–D) of the 2nd internode and culm lodging resistance index (E–H) of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. * Indicates significance at 0.05.
Figure 5. Effect of paclobutrazol on the breaking strength (A–D) of the 2nd internode and culm lodging resistance index (E–H) of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. * Indicates significance at 0.05.
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Figure 6. Effect of paclobutrazol on lignin accumulation (A–D) in the 2nd internode of wheat culm under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
Figure 6. Effect of paclobutrazol on lignin accumulation (A–D) in the 2nd internode of wheat culm under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
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Figure 7. Effect of paclobutrazol on the PAL (A,B), CCR (C,D), 4CL (E,F), and CAD (G,H) enzyme activities in 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
Figure 7. Effect of paclobutrazol on the PAL (A,B), CCR (C,D), 4CL (E,F), and CAD (G,H) enzyme activities in 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
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Figure 8. Effect of paclobutrazol on relative expression levels of TaPAL (A,B), TaCCR (C,D), Ta4CL (E,F), and TaCAD (G,H) in the 2nd internode of wheat stems under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
Figure 8. Effect of paclobutrazol on relative expression levels of TaPAL (A,B), TaCCR (C,D), Ta4CL (E,F), and TaCAD (G,H) in the 2nd internode of wheat stems under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. Error bars represent standard errors, and different letters represent significance at 0.05.
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Figure 9. Effect of paclobutrazol on the contents of ZT (A,B), GA7 (C,D), IAA (E,F), and ABA (G,H) in the 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. * Represents significance at 0.05, error bars represent standard errors.
Figure 9. Effect of paclobutrazol on the contents of ZT (A,B), GA7 (C,D), IAA (E,F), and ABA (G,H) in the 2nd internode of wheat under shading condition. NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application. * Represents significance at 0.05, error bars represent standard errors.
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Figure 10. Correlation analysis of basal internode length proportion (BILP), filling degree (FD), lignin content (LC), breaking strength (BS), culm lodging resistance index (CLRI), and lodging rate (LR) (A); section modulus, lignin content, and yield (B). JS, jointing stage; AS, anthesis stage; DS, dough stage. ** represent significance at 0.01.
Figure 10. Correlation analysis of basal internode length proportion (BILP), filling degree (FD), lignin content (LC), breaking strength (BS), culm lodging resistance index (CLRI), and lodging rate (LR) (A); section modulus, lignin content, and yield (B). JS, jointing stage; AS, anthesis stage; DS, dough stage. ** represent significance at 0.01.
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Table 1. ANOVA of spikes number per m2, grain number per spike, 1000-grain weight, grain yield as affected by cultivar, shading, paclobutrazol, and their interaction.
Table 1. ANOVA of spikes number per m2, grain number per spike, 1000-grain weight, grain yield as affected by cultivar, shading, paclobutrazol, and their interaction.
Trait/
Source of Variation		SNPGNSTGWGY(+net)GY(−net)
2020–20212021–20222020–20212021–20222020–20212021–20222020–20212021–20222020–20212021–2022
Cultivar (C)*****************************
Shading (S)******************************
Paclobutrazol (P)******************************
C × Snsns******nsns*********
C × Pnsnsnsnsnsnsnsnsnsns
S × Pnsnsns*nsnsnsnsnsns
C × S × Pnsnsnsnsnsnsnsns*ns
SNP, spikes number per m2; GNS, grain number per spike; TGW, 1000-grain weight; GY, grain yield; +net, with support nets; −net, without support nets; C, cultivar; S, shading; P, paclobutrazol. ns, not significant p > 0.05. *, ** and *** represent significance at 0.05, 0.01, and 0.001, respectively.
Table 2. Lodging behavior of wheat under various treatments.
Table 2. Lodging behavior of wheat under various treatments.
CultivarTreatments2020–20212021–2022
Time of
Lodging		Lodging
Angle (°)		Lodging Rate
(%)		Time of
Lodging		Lodging
Angle (°)		Lodging Rate
(%)
YZMNLEA (GS60)55.64b62.96cAS(GS65)49.73c71.11c
NPED (GS82)32.27c34.81dED (GS82)35.50d35.92d
SHJS (GS39)74.00a90.74aJS (GS39)82.21a98.55a
SPLH (GS55)63.12b78.89bEA (GS60)64.53b83.33b
S28NL--0c0c--0c0c
NP--0c0c--0c0c
SHAS (GS65)30.39a35.55aAS (GS65)29.65a45.18a
SPDS (GS85)17.73b21.85bDS (GS85)10.33b26.66b
Analysis of variance
Cultivar (C)--******--******
Shading (S)--******--******
Paclobutrazol (P)--******--******
C × S--nsns--**ns
C × P--*****--ns**
S × P--nsns--**ns
C × S × P--****--**
NL, natural light with no PP333 application; NP, natural light with PP333 application; SH, shading with no PP333 application; SP, shading with PP333 application; EA, early anthesis stage; ED, early dough stage; JS, jointing stage; LH, late heading stage; AS, anthesis stage; DS, dough stage; C, cultivar; S, shading; P, paclobutrazol. Data in the same column with different letters represent significance at p < 0.05 among treatments. ns, not significant p > 0.05. *, ** and *** represent significance at 0.05, 0.01, and 0.001, respectively.
Table 3. ANOVA of plant height, gravity center height, basal internode length proportion, lignin content as affected by cultivar, shading, paclobutrazol, and their interaction.
Table 3. ANOVA of plant height, gravity center height, basal internode length proportion, lignin content as affected by cultivar, shading, paclobutrazol, and their interaction.
Trait/
Source of Variation		PHGCHBILPLC
2020–20212021–20222020–20212021–20222020–20212021–20222020–20212021–2022
Cultivar (C)************************
Shading (S)************************
Paclobutrazol (P)************************
C × S******ns**nsns***ns
C × P*****nsnsnsnsnsns
S × P***nsnsnsns**ns
C × S × P*ns*nsnsnsnsns
PH, plant height; GCH, gravity center height; BILP, basal internode length proportion; LC, lignin content; C, cultivar; S, shading; P, paclobutrazol. ns, not significant p > 0.05. *, ** and *** represent significance at 0.05, 0.01, and 0.001, respectively.
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Peng, D.; Xu, H.; Guo, Z.; Cao, W.; Zhang, J.; Liu, M.; Wang, X.; Tang, Y.; Cai, T. Characteristics of Lodging Resistance of Wheat Cultivars from Different Breeding Decades as Affected by the Application of Paclobutrazol Under Shading Stress. Agronomy 2025, 15, 1848. https://doi.org/10.3390/agronomy15081848

AMA Style

Peng D, Xu H, Guo Z, Cao W, Zhang J, Liu M, Wang X, Tang Y, Cai T. Characteristics of Lodging Resistance of Wheat Cultivars from Different Breeding Decades as Affected by the Application of Paclobutrazol Under Shading Stress. Agronomy. 2025; 15(8):1848. https://doi.org/10.3390/agronomy15081848

Chicago/Turabian Style

Peng, Dianliang, Haicheng Xu, Zhen Guo, Wenchao Cao, Jingmin Zhang, Mei Liu, Xingcui Wang, Yuhai Tang, and Tie Cai. 2025. "Characteristics of Lodging Resistance of Wheat Cultivars from Different Breeding Decades as Affected by the Application of Paclobutrazol Under Shading Stress" Agronomy 15, no. 8: 1848. https://doi.org/10.3390/agronomy15081848

APA Style

Peng, D., Xu, H., Guo, Z., Cao, W., Zhang, J., Liu, M., Wang, X., Tang, Y., & Cai, T. (2025). Characteristics of Lodging Resistance of Wheat Cultivars from Different Breeding Decades as Affected by the Application of Paclobutrazol Under Shading Stress. Agronomy, 15(8), 1848. https://doi.org/10.3390/agronomy15081848

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